Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Disposal of industrial chemicals is a problem on a worldwide scale. Traditionally, toxic chemicals, such as halogenated organic compounds (HOCs) were disposed of by burying containers in the ground or simply dumping liquids onto the ground. However, these methods of disposal are unsatisfactory; buried containers sometimes degrade, leaking their contents into the environment, and liquids dumped on the ground tend to seep into the soil, eventually finding their way into water systems, thereby to contaminate the environment and domestic water supplies. This is especially undesirable in the case of harmful and/or toxic chemicals.
Recently, there have been attempts to deal with harmful waste chemicals, such as HOCs, by transforming them into less harmful products using a suitable reductive process (Chuan-Bao Wang et al., Environmental Science & Technology, 1997, vol. 31, no. 7, 2154-2156; U.S. Pat. No. 5,857,810). Destruction of HOCs by zero-valent metals, particularly iron, represents an excellent technology for environmental remediation (P. G. Tratnyek, Chem. Ind., 1996, 13, 499-503). It has been shown that granular iron can degrade many HOCs, including chlorinated aliphatics (R. W. Gillham et al., Ground Water, 1994, 32, 958-967), chlorinated aromatics (C. B. Wang et al., Proceeding of the 15th Meeting of North American Catalysis Society, Chicago, May 18-23, 1997) and polychlorinated biphenyls (F. W. Chuang et al., Environ. Sci. Technol., 1995, 29, 2460-2463), as well as nitroaromatic compounds.
The use of granular iron has, however, been problematic because of the relatively low reactivity of iron granules. To circumvent this problem, nanoscale zero-valent iron (ZVI) has been used as an efficient means for remediation of contaminated water (Chuan-Bao Wang et al., Environmental Science & Technology, 1997, vol. 31, no. 7, 2154-2156; U.S. Pat. No. 5,857,810). Nanoscale ZVI is more reactive than granular ZVI because of its high surface area to volume ratio. Typically, a colloidal suspension of nanoscale ZVI is contacted with water contaminated with HOCs (see, for example, D. W. Elliott, Environ. Sci. Technol., 2001, 35, 4922-4926). The nanoscale ZVI may be added in slurry reactors for the treatment of contaminated soil and sediment, or injected into contaminated groundwater under gravity-feed conditions. An advantage of colloidal suspensions of nanoscale ZVI is that the nanoparticles can “flow” to some extent with groundwater, reaching areas of contamination inaccessible by conventional methods. Alternatively, the nanoscale ZVI may be anchored onto granular activated carbon and other media.
Unlike granular ZVI, nanoscale ZVI is not commercially available. Generally, it is synthesized by reduction of an aqueous solution of ferric iron (Fe3+) using sodium borohydride. This produces nanoscale ZVI having a primary particle size of 1-200 nm. The nanoscale ZVI made by this procedure may, optionally, be coated with a layer of Pd by further reaction with an ethanolic solution of [Pd(C2H3O2)2]3 (Chuan-Bao Wang et al., Environmental Science & Technology, 1997, vol. 31, no. 7, 2154-2156). Nanoscale ZVI coated with Pd has also been shown to be an effective means for remediation of contaminated materials.
Other nanoscale zero-valent metals are potentially useful in other applications. For instance, nanoscale zero-valent phosphorus has potential applications in the semiconductor industry.
A disadvantage of preparing nanoscale ZVI by sodium borohydride reduction is the cost of sodium borohydride. Commercial grade sodium borohydride costs about $90/kg. Moreover, there are only a few places in the world that manufacture sodium borohydride. Consequently, the cost of nanoscale ZVI is relatively high, in some cases, too high to be commercially viable.
A further disadvantage of using borohydride to produce zero-valent metals is that borohydride is relatively unstable, meaning that its production, transport and usage require careful control, and thereby further expense.
A further disadvantage of the borohydride reduction method is that it produces large quantities of explosive hydrogen gas. Notwithstanding the inherent hazards of hydrogen gas, the additional safety protocols required for dealing with the gas on a large scale contribute to the high cost of presently available nanoscale ZVI.
A reductant thus circumventing the above-mentioned limitations would seem preferable. The reduction of Fe(III) ions to Fe(II) ions using dithionite anion is known. However, the reaction product Fe2+ has a lower redox potential than ZVI, and is limited to chemistry in solution.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.